How to know how many requests are still being processed by express?

I am trying to find a way to know if there are requests pending on the server so I can wait for them to finish in order to gracefully shutdown the server. What I have tried so far is:

var express = require("express");
var app = express();

var count = 0;
app.use(function(req, res, next){
    count++;
    console.log(count);
    next();
    res.on("end", function(){ //PROBLEM HERE! This event does not exists!!!
        count = count - 1;
        console.log(count); 
    });
});

app.get("/", function(req, res, next){
    res.send("Ok!");
});

process.on('SIGINT', function () {
    if(count === 0) process.exit();
    else console.log("Can't shutdown! Pending requests...");
});

var server = app.listen(8021);

So what is the best way to handle this case??? I am stucked.

EDIT: It was supposed to be quite an easy task. Acording to http://nodejs.org/api/net.html#net_server_close_callback

Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a 'close' event. Optionally, you can pass a callback to listen for the 'close' event.

I can simply do:

process.on('SIGINT', function () {
    console.log("Exiting");
    server.close();
    server.on("close", function(){
        console.log("Exited");
        process.exit(0);
    });
});

But my callback for on close is never called. Also tried:

process.on('SIGINT', function () {
    console.log("Exiting");
    server.close(function(){
        console.log("Exited");
        process.exit(0);
    });
});

But it doesn't work either.